1. Field of the Invention
This invention relates to a transparent glass-ceramic article having high hardness and mechanical strength.
2. Description of the Prior Art
A transparent article used in cover glasses of watches and the like is required to have high hardness, mechanical strength and transparency. From the viewpoint of transparency, materials used for such an article are limited to plastics, glass and transparent crystalline materials such as sapphire. The plastics and glass lack hardness. The transparent crystalline materials lack strength or require time and labor for molding and processing, and the cost of production becomes very high. For this reason, chemically strengthened glass is used in general-grade articles at the sacrifice of hardness, and in some luxurious articles, sapphire single crystal is used at the sacrifice of cost.
Chemical strengthening of glass-ceramic by ion exchange has been proposed in Japanese Patent Publication No. 49299/1972 (corresponding to U.S. Ser. Nos. 365035, 365117, 365177, 365178, 365198, 365199, 365201 and 365202). In some of the glass-ceramic articles desribed in the prior art, no consideration is given to their transparency. Or some of them require high treating temperatures in order to exchange Li.sup.+ in the glass-ceramic with Na.sup.+ or K.sup.+, and therefore, nitrates such as potassium nitrate having a relatively low heat decomposition temperature cannot be used as a salt bath. Strength cannot be improved unless a salt bath of a sulfate or chloride which has a high heat-decomposition temperature but is strongly corrosive is used. In articles whose transparency is important as in cover glasses for watches, the use of a strongly corrosive molten salt (such as KCl, K.sub.2 SO.sub.4, NaCl or Na.sub.2 SO.sub.4) is not desirable. The ion exchange treatment will corrode the surface layer and reduce its transparency, and if the surface layer is polished in order to obtain transparency, the resulting surface compressive stress layer will be removed and the strength of glass decreases to that before the ion exchange.